This invention relates generally to hydraulic systems and controls and is more particularly concerned with a rotary control valve for simultaneously controlling fluid distribution to a plurality of hydraulically actuated devices (e.g., piston-type actuators) and with a system incorporating such a valve.
The invention is especially useful in connection with hydraulic actuator arrays for use in parts fixtures, such as vises, for machine tools. These parts fixtures may, for example, utilize hydraulically controlled mechanical stops to establish the proper position of a workpiece for machining. The stops may be actuated manually or, in the case of computer-controlled systems, automatically.
Electrically operated solenoid valves are among the most reliable means of automatic actuation currently in commercial use. Solenoid valves offer the advantage of being substantially leakage-free since constant pressure is maintained on the valve seal. However, a single solenoid valve is only useful to control one actuator in the ordinary case. Solenoid valves also tend to make hydraulic control systems bulky and impractical, especially when complex valving arrangements are involved.
Rotary valves provide an alternative to solenoid valves but have likewise been of only limited utility, due particularly to problems associated with internal leakage (which causes actuator creep), control logic complexity, and equipment size limitations, among other factors. U.S. Pat. No. 3,021,869 issued Feb. 20, 1962 to Ross is exemplary in this regard. Ross discloses a rotary valve for controlling a double acting hydraulic piston. The valve has a hollow cylindrical casing with four fluid ports: a supply port positioned at one end of the casing and connected to receive fluid pumped from a reservoir, two load ports aligned longitudinally of the casing with the supply port and connected to opposite sides of the piston housing, and a reservoir port disposed between the load ports and connected to the reservoir. The valve casing houses a rotary plug with internal passages that may be brought into alignment with the various casing ports to control fluid flow to and from the opposite sides of the piston housing. Circumferential ring seals on the valve plug seal with the valve casing between each load port and the reservoir port.
As the ring seals in the Ross valve wear during use, internal leakage of high pressure fluid to the load ports develops and causes the piston to creep. Hydraulic engineers often avoid rotary valves for this reason alone. Additionally, however, because the Ross valve requires three discrete positions to control a single piston, modification to accommodate more than one piston would complicate the valve structure considerably and could lead to logic error conditions in which a piston (or pistons) is improperly extended or retracted. The Ross valve suffers from yet another drawback in that a large thrust bearing is necessary in practice to support the valve plug within the casing. More particularly, because the valve plug is made shorter than the casing so as to provide a fluid supply chamber at one end of the casing, the plug will experience high thrust loads due to the pressure of the supply fluid, thus necessitating a thrust bearing of substantial construction.
Due to the above-discussed and other problems related to hydraulic control, the automation of parts fixtures has remained impractical for the vast majority of machine shops.